Members of the genus Bison are large even-toed ungulates within the subfamily Bovinae. Two extant species and four extinct species are recognized. The surviving species are the American bison, Bison bison (with two subspecies, the plains bison, Bison bison bison, and the wood bison, Bison bison athabascae), found in North America, and the European bison, or wisent (Bison bonasus), found in Europe and the Caucasus. While these species are usually grouped into their own genus, they are sometimes included in the closely related genus Bos, together with cattle, gaur, kouprey and yaks, with which bison can interbreed.